Hi Keith,
I build planes but I always put my decals on before weathering. Otherwise your tank will be all weathered with shiny new decals!
My order (remembering I build planes) is paint including any effects like varying colour on panels etc, coat of future, decals, panel line wash, maybe some pastels maybe a little more airbrushing, and then a flat coat of lacquer. I try to weather over the decals as well so they blend in to whatever degree of weathering I apply.